I'm so sorry, I lost one of my first bettas that way.
He sure was a lovely boy since you talked so fondly of him.
They can fit through the tiniest cracks so don't blame yourself over it, it's just in their nature to jump.
Try covering the hole with mesh and some glue, you'll still be able to drop pellets in for your next fish if you decide to get one.

I have glass tops on all of my tanks, so usually jumping isn't a problem. But just that one moment I guess, when I wasn't looking .... At least I have my other guys and the sorority girls. They're keeping my spirits up. They are the reason I keep going.